[unreadable] Recent years have seen important developments in the study of hearing, especially the emergence of molecular genetics to probe the underlying mechanisms of hearing loss. It is estimated that 1 in every 1,000 newborns is profoundly deaf, while nearly 1 in 20 has a significant hearing impairment. In more than half of these cases, the cause is genetic. As of February 2002, twenty-nine specific genes have been associated with different forms of nonsyndromic human deafness. In addition, many other gene products involved in normal cochlear function have been identified. The foreign collaborator (Elgoyhen) cloned and characterized two novel nicotinic receptor genes (alpha 9 and alpha 10) that are expressed in cochlear hair cells. Alpha 9 and 10 encode receptor proteins that mediate the effect of acetylcholine (ACh) released by efferent neurons onto cochlear hair cells. The Principal Investigator (Fuchs) showed that calcium entry through the ACh receptor leads to hair cell hyperpolarization, reducing transmitter release to cause a loss of tuning and sensitivity in the auditory nerve fibers. This FIRCA proposal brings together the expertise of the Fuchs and Elgoyhen laboratories to conduct a series of electrophysiological and molecular genetic experiments to elucidate the properties and function of these cholinergic receptors in hair cell physiology. The long-term goal of this proposal is to define the physiological role of the hair cell's cholinergic receptor. In addition, it is expected that the results obtained will contribute to our understanding of the role of efferent cholinergic input in the genesis and potential treatment of hearing impairment produced by loud sound or ototoxic drugs. The immediate aims of this application are three-fold: first, the pharmacological and physiological comparison of recombinant alpha9/alpha10 receptors expressed in Xenopus laevis oocytes with native cholinergic receptors of inner and outer hair cells in acute cochlear explants; second, the characterization of the ontogeny of cholinergic responses in inner hair cells and third, the physiological analysis of synaptic contacts onto inner and outer hair cells in mice with genetic modifications in the Acra9 gene, namely the alpha9 null mutant mouse and a knock-in mouse bearing a gain of function mutation. This research will be done primarily in Argentina at INGEBI (National Research Council) in collaboration with Ana Belen Elgoyhen as an extension of NIH grant #R01 DC01508. [unreadable] [unreadable]